1. Field of the Invention
This invention relates to photovoltaic devices which convert solar energy into electrical energy, and to a method for manufacturing such devices.
2. Description of the Prior Art
Solar energy will increasingly be seen as advantageous from the standpoint of providing environmental safeguards and as compensation for the use of natural resources which cause unacceptable levels of pollution. Photovoltaic devices which directly convert solar energy into electrical energy are attractive because it is easy to invest in the production of solar cells in accordance with the consumption of electric power.
In order to prepare for increasing needs, the photovoltaic devices need improved characteristics, particularly conversion efficiency (an evaluation standard for photovoltaic devices). Improved conversion efficiency will reduce the cost difference between the photovoltaic devices and commercial electrical supplies.
There are many ways to improve the characteristics of the photovoltaic devices, for example, by developing the photovoltaic device structures and researching superior compositions of materials. FIG.12 shows a cross-sectional illustration of a prior art photovoltaic device which mainly comprises amorphous silicon (hereinafter "a-Si"). In FIG.12, 141 is a transparent insulating substrate made from a material such as plate glass or silica glass. 142 is a transparent conductive electrode layer, comprised of SnO.sub.2 or ITO. 143 is a photovoltaic layer including: a) a p-type semiconductor layer 143p, which comprises an amorphous silicon carbide (hereinafter "a-SiC") layer, b) a buffer layer 143b, c) a photosensitive layer 143i comprised of an intrinsic a-Si, and d) an n-type semiconductor layer 143n comprised of an n-type a-Si. 144 is a back electrode composed of aluminum.
There are several ways to improve the conversion efficiency by changing the structure. For example, the buffer layer 143b in the photovoltaic layer 143 may provide improved contacting conditions between the p-type semiconductor layer 143p and the intrinsic semiconductor layer 143i; thus, improvements in the buffer layer 143b can prevent carrier loss due to recombination of charge carriers.
Another way to improve the photovoltaic device is to alter a surface of the transparent conductive electrode layer 142. This surface is usually formed so as to be uneven. Therefore, any light which passes through the transparent substrate is dispersed by the uneven surface, even if the light approaches the surface vertically. The light is bent due to the dispersion, which makes the light path longer. As a result of the longer light path, the probability of the light being absorbed within the photovoltaic layer 143 is increased, and the conversion efficiency may be improved.
In the prior art photovoltaic devices, this characteristic improvement, caused by the uneven surface, is easy to manufacture. On the other hand, improvement of the interface quality to prevent the recombination of charge carriers is not achieved only by the improvement of the quality of the semiconductor layer interface. To improve semiconductor layer quality, it is necessary to perform an adjustment between the semiconductor layers. For consistency between photovoltaic devices, it is necessary to improve the quality of the interfaces between the semiconductor layers.
Another approach is to provide optimum optical bandgaps in the photovoltaic devices. Amorphous semiconductors having large bandgaps and high photo-conductivity are needed. For example, an amorphous semiconductor with a large optical bandgap may be composed of an a-Si layer with carbon (C) or oxygen (O) added to produce a-SiC or a-SiO, a-SiC's and a-SiO's are easy to form by adding methane (CH4) or carbon dioxide (CO2) to silane gas (SiH4) which is the main plasma CVD reaction gas for producing a silicon semiconductor layer. This plasma-CVD method is the same method used to form an a-Si layer, which substantially comprises the main structure in the photovoltaic devices. Therefore, the method has an advantage of allowing continuous formation of succeeding layers of a-Si and a-SiC (or a-SiO) during the manufacturing of the photovoltaic devices.
A method of forming amorphous semiconductor layer including carbon (C) or oxygen (O), is described in Journal of Non-Crystalline Solids 97 & 98 (1987), p 1027-1034, which can increase the optical bandgap. However, this method causes a reduction in the photo-conductivity, and thus, it is difficult to obtain superior photovoltaic characteristics.
Another method of increasing the optical bandgap is by increasing the hydrogen concentration in the amorphous semiconductors. For the a-Si layers formed by plasma decomposition of silane gas (SiH4), hydrogen is incorporated in the a-Si layers at a high degree. Therefore, this method can widen the optical bandgap. This method may be easily performed by reducing the substrate temperature during the manufacture of amorphous semiconductors.
On the other hand, increasing the optical bandgap by incorporating large amounts of hydrogen causes a reduction in the quality of the semiconductor. This is due to the reduced substrate temperature during manufacture, which makes it difficult to obtain high photo-conductivity characteristics in the photovoltaic devices.